J. Am. Chem. SOC.1993,115. 5284-5285
5284
Communications to the Editor [l.l]Paracyclophane. Spectroscopic Evidence for
Scheme 1"
Generation of the Bis(methoxycarbony1) Derivative Takashi Tsuji,' Masakazu Ohkita, and Shinya Nishida
Department of Chemistry, Faculty of Science Hokkaido University, Sapporo 060, Japan Received March I I , I993 [2,2]Paracyclophane was first isolated in 1949,' and its structure, in which benzene rings are bent and aligned in parallel in close proximity, fascinated chemists. Despite the extensive exploration of the chemistry of cyclophanes2 and considerable interest in strained c o m ~ p o u n d s ,the ~ lower [1.2] and [1.1] homologues have remained unknown. We report here the photochemical generation of the first [ 1.l]paracyclophane derivative 2b from l b and the results of IH N M R and UV/vis spectroscopic studies.
a, R = H ; b, R
=
9)
12
c::
y&ph
(a) LAH and then PBr3; (b) AcOt-Bu/LDA and then CF3C02H; (c) ( C F ~ C O ) ~ O / B F ~ O E ~ ~ / C(d) H hv, ~ CCzH2/acetoneXH2C12; ~Z; (e) HC02Et/NaOEt and then TsN3/EtsN; ( f ) hv in MeOH; ( 9 ) LDA and then PhSeBr; (h) H202.
CQ2CH,
Synthesis of bis(Dewar benzene) compound l b was achieved as outlined in Scheme I. The (CF&0)20-induced double cyclization of 6,which was prepared from 44in five steps, cleanly afforded 7. The photocycloaddition of acetylene to 7 stereoselectively occurred to produce 8 as the single diacetylene [2 + 21 adduct. The anti stereochemistry assigned to 8 on the basis of N O E experiments was later confirmed by the same experiments on lb.5 The subsequent conversion of 8 into l b was accomplished according to the procedure previously reported for the preparation of Dewar benzene derivatives.6 Compound l b was obtained in about 1% yield overall in 11 steps from 4. Irradiation of l b in EPA glass7 with a 254-nm light source at 77 K led to the development of absorption showing A, a t 256, 348, and 405 nm and extending to 480 nm (Figure 1, spectrum A). The generated species was photochemically labile and efficiently consumed when irradiated with >390-nm light, which was not absorbed by l b (spectrum B). The difference spectrum between spectra A and B indicated that the initial product was converted to the species exhibiting A, at 267 nm (spectrum C). (1) (a) Brown, C. J.; Farthing, A. C. Nature 1949, 164, 915-916. (b) Brown, C. J. J. Chem. SOC.1953, 3265-3270. (2) (a) Cram, D. J.; Cram, J. M. Acc. Chem. Res. 1971,4,204-213. (b) Cyclophanes; Keehn, P. M., Rosenfeld, S. M., Eds.;Academic Press: New York, 1983. (c) Cyclophanes I and II (Topics in Current Chemistry I I 3 and lIS);V6gtle, F.,Ed.;Springer-Verlag: Berlin, 1983. (d) Vogtle, F. CyclophanChemie; B. G. Teubner Verlag: Stuttgart, 1990. (3) (a) Greenberg, A,; Liebman, J. F. Strained Organic Molecules; Academic Press: New York, 1978. (b) Wiberg, K. B. Angew. Chem., Int. Ed. Engl. 1986, 25, 312-322. (c) Synthesis of Non-natural Products: Challenge and Reward (Tetrahedron Symposia-in-print 26); Eaton, P. E., Ed.; Tetrahedron 1986,42, 1549-1915. (d) The July/August 1989 issue of Chemical Reuiews reports exclusively on strained organic compounds. (4) Jones, D. G. Ger. Patent 950 285,1956; Chem. Abstr. 1959,53, 17926. ( 5 ) According to force field calculations, the central six-membered ring in the mono adduct 12 is preferentially bent toward the cyclobutene ring as depicted. Prevailing steric factors would thus guide the entry of the second acetylene anti to the first one. The calculations were performed with Chem3D Plus, Ver. 3.0; Cambridge Scientific Computing: Cambridge, MA, 1990. (6) Tsuji, T.; Komiya, Z.; Nishida, S . Tetrahedron Lett. 1980,21, 35833586. (7) A 5:5:2 mixture of ether, isopentane, and ethanol.
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t-
-o*2v
......... ..
-0.4
-0.6
I
250
300
350
400
450
c
500
/ nm
Figure 1. Difference UV/vis spectra (A) between the absorptions before and after the irradiation of Ib and (B) between those before and after further irradiation with>390-nmlight (seetext for conditions). Spectrum C was obtained by adding spectrum B to spectrum A.
Table I. 'H NMR Parameters for lb, Zb, and 3b@ species HA(&) JAB(Hz) H B / H c ( ~ J
B~W HD/HE(G)
lb 2b 3b
2.4 8.8 6.4
7.19 7.60 7.12
